Increasingly stringent legal requirements with regard to permissible pollutant emissions of automobiles in which internal combustion engines are fitted, make it necessary to keep the pollutant emissions as low as possible when the internal combustion engine is running. On the one hand, this can be achieved by reducing pollutant emissions which are produced during the combustion of the air/fuel mixture in the respective cylinder of the internal combustion engine. On the other hand, exhaust gas aftertreatment systems that convert the pollutant emissions produced during the air/fuel mixture combustion process in the respective cylinders into harmless substances, are used in internal combustion engines. Catalytic converters, which convert carbon monoxide, hydrocarbons and nitrogen oxide into harmless substances are used for this purpose. The specific effects of the production of pollutant emissions during combustion, and the highly efficient conversion of the pollutant components by a catalytic converter, both demand a very precisely-set air/fuel ratio in the respective cylinder.
With regard to an actual, suitably-low pollutant emission while the vehicle is being operated, requirements regarding a diagnosis of various components assigned to the internal combustion engine play an increasingly important role. For example, it is stipulated in a draft of the California Air Regulation Board (CARB 1968.2 Annex A(e), 6.2.1 (C), of Sep. 8, 2006) that an imbalance in an air/fuel ratio in one cylinder or in several cylinders, due to a cylinder-specific malfunction, that may be present in the region of the fuel injection valve for example, must be detected.